1. Field of the Invention
The present invention relates to polyvinyl alcohol (hereinafter abbreviated to PVA) fibers having a flattened cross-sectional profile. The fibers are capable of being readily fibrillated. The present invention further relates to a nonwoven fabric comprising the fibers, and to a fibrillated fabric prepared by applying high shear force to the nonwoven fabric.
2. Discussion of the Background
Fibrillated PVA fibers are produced according to a general method that comprises mixing and spinning PVA with other polymer, oil, fat or surfactant immiscible with PVA to make the resulting fibers have a sea-island structure followed by splitting the structure at the interface thereof to give split fibers. For example, a technique has been proposed for it, and is as follows: a PVA polymer is dissolved in a solvent along with other polymer miscible with vinyl alcohol polymer, for example, polyacrylonitrile and/or its copolymer, polymethyl methacrylate, cellulose polymer, starch and the like to form a phase-separated structure in the resulting mixture, then the mixture serving as a spinning solution is wet-spun to give fibers having a sea-island structure, and the fibers are beaten into fibrillated fibers (e.g., see JP-A 49-10617, JP-A 51-17609, JP-A 8-284021, JP-A 8-296121, JP-A 8-81818, JP-A 10-102322, JP-A 10-219515, JP-A 10-219517, JP-A 10-237718).
However, in order to attain sufficient fibrillation in the above-mentioned method, the PVA polymer content of the polymer mixture must be substantially from 30 to 70% by mass. Accordingly, the PVA polymer content of the fibers obtained is low, and the fibers would lose the intrinsic properties of PVA polymer, such as chemical resistance, hydrophilicity, weather resistance and high tenacity. In general, PVA fibers are formalated for making them resistant to water, but the process is problematic in that the fibers are degraded through hydrolysis with strong acid or alkali used for the treatment. When PVA fibers are formalated along with cellulose polymer, it is further problematic in that the polymer mixture is much crosslinked at the interface of PVA polymer/cellulose polymer and, as a result, the fibrilability of the resulting fibers is significantly lowered.
Similarly, a liquid substance such as oil and/or surfactant is dissolved in a solvent along with a PVA polymer to form a liquid mixture having a phase-separated structure, then the resulting mixture serving as a spinning solution is spun in wet into sea-island structured fibers in which the island component is formed of the liquid substance, and the fibers are beaten into fibrillated fibers. According to the method, however, the liquid substance to be added must be at least 30% by mass in order that the fibers produced could be fibrillated. As a result, the liquid substance may flow out in the coagulation bath in the process of wet-spinning, and it may contaminate the bath. For this reason, the industrial production of the fibrillated fibers according to the method is difficult. In addition, a major part of the liquid substance flows out in the coagulation bath, therefore the retention of the substance in the final product is low, and the fibrillation of the fibers is not enough.
On the other hand, for obtaining splittable fibers in a process of melt-spinning different types of polymers that are alternately aligned, for example, a technique of spinning a combination of a PVA polymer and a polyester polymer to give splittable fibers has been proposed (e.g., see JP-A 2001-11736). However, the melt-spinnable PVA polymer is readily soluble in water and is therefore poorly resistant to water, and, in addition, it could not be formalated for improving its water resistance. Accordingly, it is impossible to obtain fibrillated PVA fibers in a process of spinning multiple components in melt.